1. Field of the Invention
The present invention relates to magnetic toner used in an electrophotographic image formation method.
2. Description of the Prior Art
In many cases, image formation for copiers, printers, etc., employs electrophotography in which a latent electrostatic image is formed on a charged photoreceptor surface, then developed with a developing agent. The photoreceptor surface is charged by a corona discharge, or by a conductive roller or other means, and a latent image is formed by exposure to light emitted from a semiconductor laser, an LED array, or other light sources.
Magnetic brush development is generally employed. A developing agent supplied to a developing roller (consisting of a non-magnetic sleeve and a permanent magnet member incorporated therein) opposed to the photoreceptor surface is conveyed to the developing region by, for instance, rotating the sleeve. An electrostatic latent image is visualized by sliding a magnetic brush formed on the sleeve on the image bearing surface (photoreceptor surface) such that the former frictionally contacts the latter. Then, a toner image is transferred onto, for instance, a plain sheet and fused thereon to become a final image.
Two types of developing agents are known: a two-component developing agent including toner and carrier as main components, and a one-component developing agent including toner as a main component but not carrier. In many cases, each of the two types employs magnetic toner including a binding resin and magnetic powder as main components.
As magnetic characteristics, the magnetic toner must have a large saturation magnetization, particularly, when it is used in the one-component developing agent, because magnetic brush filaments must be high. Further, magnetic toner should have a large coercive force to provide superior developing agent transfer, flow, and cohesiveness. It is desirable that magnetic toner provide a solid black color alone, or with least amounts of coloring agents added.
To satisfy the above magnetic characteristics, the requirement of a solid black color, and other factors, magnetic toners in current use generally include magnetite (Fe.sub.3 O.sub.4) as a magnetic powder. In general, magnetite for this purpose has a saturation magnetization (.sigma..sub.s) of 60-90 emu/g and a coercive force (iHc) of 50-400 Oe.
However, when conventional magnetic toner having the above composition is used as the developing agent alone or with a magnetic carrier, although it can provide a sufficient image density, resolution, etc., black traces may occur due to trailing at the edges of an image, a phenomenon called "tailing." This phenomenon is particularly marked in sleeve rotation development.